Surmountability, agility, and stability are required for a moving mechanism such as a robot having wheels. The surmountability of a robot having wheels means the ability of a robot which can move without being overturned or in an inoperable state during moving while maintaining a weight balance of the robot body stable in a state where a wheel is in contact with a road surface having any shape. In addition, the agility in the robot having wheels means ability of the robot which can realize spatial and temporal efficiency and diversification with respect to the operation of the robot such as lightness of a direction changing, a movement or a motion in a state where a posture is maintained. It is desirable to design an optimum structure so as to enable operation with a realistic cost while maintaining balance during these performances.
In a four-wheel structure mechanism 100 of the related art shown in FIG. 16, two driving wheels 101 which are separately driven in the same straight line are disposed on the front side, and caster wheels 102 which can freely rotate in a horizontal surface are disposed behind the two driving wheels 101. In the structure of the related art, at least three wheels among the four wheels can travel on a road surface having various unevenness while being in contact with the road surface, and a certain level of surmountability is realized.